1. Technical Field
The present invention relates to a high-strength conductive thermoplastic resin formed product comprising a composite mixture of long glass fibers, a conductive material and a thermoplastic resin and a manufacturing method thereof.
The fiber-reinforced conductive thermoplastic resin formed product (high-strength conductive thermoplastic resin formed product) of the present invention is suitably applicable particularly for an electrostatic scattering material or an electromagnetic wave shielding (EMI shielding) material.
2. Background Art
A conductive thermoplastic resin comprising a composite mixture of a thermoplastic resin with a conductive material can be formed into various complicated shapes by injection molding or injection-compression molding, and further, has such advantages as a low forming cost and possibility of mass production. In the field of conductive materials such as electrostatic scattering materials and electromagnetic wave shielding materials, therefore, conductive thermoplastic resins have now been popularly used in place of fabricated metal articles and conductive surface-fabricated articles treated with a conductive coating material or metal plating. However, these conductive thermoplastic resins had a defect of a low mechanical strength, in spite of the industrial advantages including the wide industrial applicability and the low cost as described above.
More recently, possibility of improving strength of conductive thermoplastic resins, and inventions of adding fibers reinforcing materials such as glass fiber have been proposed. However, strength of a formed product is seriously affected by the remaining fiber length after forming. Even when such a resin reinforced with long fibers longer than 1 mm issued as used as a raw material, therefore, reinforcing fibers are broken during the forming fabrication step, resulting in that most of the long fibers becoming short ones shorter than 0.2 mm, and a remarkable reinforcing effect is unavailable. Measures so far taken to improve strength of resin formed product with conventional glass fibers cannot therefore be sufficient for the purpose.
Now, prior art will be described with reference to some concrete examples.
Japanese Unexamined Patent publication No. 63-90,564 proposes a resin composition comprising a blend of a thermoplastic resin, a stainless steel fiber, and a glass fiber. In this invention, a glass fiber having a diameter within a range of from 5 to 50 .mu.m and an aspect ratio within a range of from 50 to 500 is used, and the glass fiber length corresponds to a range of from 0.25 to 25 mm. Even when the glass fiber of such a length is used, preparation of pellets for injection molding through melting and kneading of the aforesaid blend in an extruder causes the glass fibers to be broken during the extruding step into short fibers. The above patent publication proposes also a method of converting glass fibers into pellets with the use of a converging agent and using such pellets as they are for injection molding. In this method, the glass fibers are opened by an inline screw of the injection molding machine upon melting and kneading, and dispersed in the molten resin. At this point, however, shearing by the non-molten resin or the screw occurs, resulting in conversion of the long glass fibers into short ones.
Japanese Unexamined Patent Publication No. 60-18,315 proposes a conductive forming material prepared by bundling conductive fibers and inorganic fibers (glass fibers), forming a resin layer on the surface thereof, and then cutting into pellets. When conducting injection molding with the use of this material, the resin is only that formed on the surface of fiber bundle, and the glass fibers are not impregnated with a resin. Glass fibers are broken under the effect of friction between fibers or shearing by non-molten resin or the screw into short fibers.
Similarly, in Japanese Unexamined Patent Publication No. 59-23,595, Japanese Examined Patent Publication No. 62-36,090, Japanese Unexamined Patent Publication No. 60-88,064, Japanese Examined Patent Publication No. 4-68,348, Japanese Unexamined Patent Publication No. 62-132,959, or Japanese Examined Patent Publication No. 5-55,961, while glass fiber is used as a component for improving strength, melting and kneading carried out on a biaxial or uniaxial extruder during pelletizing step break the glass fibers into short ones.
As described above, it has been the conventional practice, taking account of uniform dispersion in the resin formed product, to melt and knead a composition comprising a conductive material, a thermoplastic resin and glass fibers in an extruder to produce pellets of such a composition. In this process, glass fibers are reduced in length into short fibers, and mechanical strength becomes insufficient. Or, in a method of coating the surface of a fiber bundle including glass fibers with a converging agent or a thermoplastic resin, even when a blended mixture of a thermoplastic resin, a conductive material and glass fiber bundle made by the aforesaid method is used as it is as a material for injection molding, friction between glass fibers and shearing force caused by non-molten resin or the screw break the glass fibers into short fibers, resulting in an insufficient mechanical strength.
With a view to preventing the fibers from being broken into short ones, a conceivable counter-measure is to inhibit breakage of the glass fibers during melting and kneading step such as forming.
More specifically, in general, this would be achieved by developing an improved forming apparatus and acting on forming conditions. When adopting the injection molding process for forming fabrication, for example, a conceivable measure is to develop an improved forming apparatus in which the screw design is changed into a deep-threaded one and a reduced compression ratio is used. Forming conditions for maintaining the glass fiber length include a reduced number of revolutions of the screw, a smaller shearing force, and a reduced back pressure.
The present inventors tried to develop a conductive high-strength thermoplastic resin formed product by combining these measures. As a result, although breakage of glass fibers was inhibited and long fibers tended to be maintained, a reduced kneadability resulted in insufficient dispersion of the glass fibers and the conductive material in the formed product. Accordingly, the resultant formed product had non-uniform strength, not exhibiting a uniform conductivity, and fatal defects as a formed product were observed.
From the point of view of imparting conductivity, and also of maintaining energizing channels by the conductive material and preventing occurrence of electric strain, it is important that the conductive material should be uniformly dispersed throughout the thermoplastic resin formed product. For this purpose, kneading dispersion upon forming or during preparation of the raw materials for forming would be enhanced, but breakage of reinforcing fibers occurs at this point. It is needless to mention that uniform dispersion in the thermoplastic resin formed product is important also for the reinforcing fibers from the point of view of strength. However, maintenance of long fibers upon which mechanical strength of the formed product depends, particularly long fibers longer than about 0.4 mm could never be achieved in the conventional art. Particularly, a conductive glass-fiber-reinforced thermoplastic resin formed product, available by the injection molding process or the injection-compression molding process, which contains long glass fibers longer than 0.4 mm as available in the present invention in a prescribed amount, and has both a high strength and electric conductivity, has not as yet been available.